This invention relates to a water based photosensitive composition containing a water-soluble polymer and photosensitizer.
In the conventional making of a name plate or precision part, a resist pattern is formed on a support member by a photosensitive composition which, when exposed to an active light, indicates different degrees of solubility in a solvent between the portion exposed to the active light and the portion not exposed to the active light. Depending on the object intended, the photosensitive composition is demanded to have such property as can provide a strong anticorrosive photo resist or a satisfactorily colored photo resist. In either case, a water based photosensitive composition is preferred which excels in promoting labor hygienics and reducing cost.
Macro molecular substance (polymer) hitherto used in the production of the above-mentioned water based photosensitive composition include natural macro molecular substance such as gelatin, casein, fish glue, and egg albumin and synthetic water-soluble macro molecular substance such as polyvinyl alcohol, polyvinyl pyrrolidone, and polyacryl amide. The photosensitivity and image-resolving property of a photosensitive composition particularly prepared from a mixture of a macro molecular substance and photosensitizer largely depend as a rule on the average molecular weight of a macro molecular substance contained in the photosensitive composition. To ensure a desired photosensitive property, therefore, it is necessary to accurately control the average molecular weight of a macro molecular substance used. The molecular weight of a refined natural macro molecular substance depends to a large extent on the molecular weight of the raw material and on the refining process. Since the raw material consists of molecules widely differing from each other in molecular weight, it is difficult to prepare a refined natural macro molecular substance of a desired average molecular weight. Thus, a photosensitive composition containing a natural macro molecular substance tends to fail to exhibit a satisfactory photosensitivity and image-resolving power. With respect to gelatin in particular which has a low solubility in water and is extremely difficult to handle at ordinary temperature, it is necessary to heat gelatin for its dissolution, application and development. Casein has to be dispersed in water by being dissolved in an alkali solution.
Casein is a globular protein which belongs to a conjugated protein (or phosphoprotein). On the other hand, gelatin is a fibrous protein belonging to a simple protein (or scleroprotein). Both gelatin and casein have a large average molecular weight. This means that gelatin and casein have a high setting point, that is, a low solubility in water, and, when applied as a photosensitive solution, have a high photosensitivity and a high dark reaction rate. Egg albumin is a globular protein belonging to the simple protein, has an average molecular weight which is not appreciably large, a high solubility in water and can be easily handled at ordinary temperature. However, the egg albumin contains a large amount of cystin due to its specific molecular structure and tends to give rise to thermal vulcanization and coagulation, and consequently is found unstable when used as a photosensitive composition. Synthetic water-soluble macro molecular substance can be produced with a prescribed average molecular weight and controlled, for example, in photosensitivity and image-resolving property with greater ease than natural macro molecular substance. However, a resist film utilizing a synthetic water-soluble resin is low in adhesiveness to a substrate and in elasticity. Thus, if the resist film is used as a photo resist, side etching is promoted. Also, the resist film mentioned fails to exhibit a sufficient resistance to corrosion, rendering it necessary to cure the resist film with anhydrous chromic acid. Further the said photo resist has an undesirably low dyeability to a hydrophilic dyestuff. This drawback can be easily inferred from the following fact. Where a water soluble macro molecular substance, synthetic or natural, is dyed, then its dyeing density largely depends on the bonding strength of the ionic bonds between a functional group contained in said macro molecular substance and a dyestuff applied. It is known that the extent of said bonding is defined by a number of functional groups contained in said water-soluble macro molecular substance. Usable hydrophilic dyes include direct, acid and basic dyes. An amino group in the macro molecular substance functions as an adsorption site for direct and acid dyes which contains a sulfonic groups in the molecule. A carboxylic group functions as an adsorption site for basic dyes which contains an amino groups in the molecule. Where comparison is made between a number of amino groups contained in a natural water-soluble macro molecular substance and that contained in a synthetic water-soluble macro molecular substance, the natural water-soluble macro molecular substance contains about 1.0 m eq of amino groups per g of macro molecular substance, while the synthetic water-soluble macro molecular substance contains about 0.1 m eq of amino groups per g of macro molecular substance. As seen from these numerical data, the synthetic water-soluble macro molecular substance can be expected to provide the amount of ionic bonding only accounting for about one-tenth of that observed in the natural water-soluble macro molecular substance.